The present invention relates generally to bevel gear pairs, and is more specifically directed to a bevel gear pair wherein a ring gear and pinion are formed having different flank surface finishes such that unmatched gears can be used in sets and still maintain acceptable noise generation limits.
A high degree of noise reduction in automobiles having offset axles is achieved using bevel gear pairs. In the conventional manufacture of bevel gear pairs consisting of a ring gear and pinion, the teeth of individual gears are cut and hardened, and then the gears are lapped. Lapping is accomplished by running the gears together, usually in the presence of an abrasive lapping compound, until an acceptable noise level for the gear pair is achieved. The configuration of these gears results in high sliding speed over the flanks of the gear teeth, thereby making these gears particularly well suited to the lapping process. A drawback associated with this method of gear production is that once the gear pair is lapped, the gears must be stocked, mounted, or replaced in matched sets.
European patent number EP 0 229 894 B1 discloses a process for manufacturing hypoid gears whereby the gears are cut by first milling, and then the gears are case hardened and lapped in batches. Pairs of ring and pinion gears are identified during lapping and must be kept together subsequent thereto. A drawback of this process is that although the lapping process improves running noise of the gear pairs, concentricity errors and local eccentricities created as a result of distortion during hardening remain. If the resulting noise level is unacceptable, other fine machining techniques must be employed, many of which are set forth in patent number EP 0 229 894 B1.
For obvious reasons, it is desirable to eliminate the necessity of having to supply bevel gear pairs in matched sets. To this end, the above-described European Patent discloses a continuous grinding process whereby distortion due to hardening, as well as concentricity errors, and eccentricities can be reduced. However, this process does not produce gears that operate at lower noise levels than those manufactured by the above-described machining, hardening and lapping process.
The difficulties in achieving the noise standards necessary for bevel gears pairs used in passenger cars will now be explained with reference to FIGS. 1-4 which show structure borne noise measurements made for a bevel gear pair ground on a spiral gear grinding machine type WNC 30 made by Oerlikon Geartec AG, Zurich. The gear pair was then tested on a Spiromat T 20 also made by Oerlikon Geartec. The graphs of FIGS. 1-4 were determined by employing a program referred to as xe2x80x9cMess Topxe2x80x9d. The acceleration level spectrum mV versus the frequency in Hz is plotted in the figures with the gear meshing frequency being approximately 320 Hz. Acceptable noise levels are those that are between the vertical bars illustrated in the figures. The operating principle upon which the graphs in FIGS. 1-4 were arrived at is based on an analysis of structure-borne noise by means of a seismic sensor. This method of analysis is described in detail in the Oerlikon company publications xe2x80x9cOerlikon Spiromatic contex T20 CNC Kegelradtester, Einflankenwxc3xa4lzprxc3xcfung und Kxc3x6rperschallanalysexe2x80x9d [Oerlikon Spiromatic contex T20 CNC Bevel Gear Tester, Single Flank Generating Testing and Analysis of Structure Borne Noise], December 1990, 9202/WA 410 935d. With respect to a ground bevel gear pair, the tooth engagement frequency, and the first and second harmonics thereof, are generally critical.
FIG. 1 shows the results for a first optimization of the gear pair tested. The tooth engagement frequency as well as the first and second harmonics lie inside of the vertical bars, that is between the lines indicating an acceptable noise level. However, on the traction or drive side of the gears, the noise levels fall outside of acceptable limits.
FIG. 2 shows that by further optimization of the gear tooth geometry, it was possible to reduce the frequency on the traction side from 60 mV to less than 20 mV, and from 50 mV on the coast side to approximately 30 m. However, there is a wide noise scatter adjacent the tooth engagement frequency, as well as near the harmonics of that frequency. Normally, this random noise is not unpleasant to the human ear. However, if, as is the case in FIG. 2, a periodicity is associated with this scatter, the generated noise can be unpleasant to the human ear.
FIG. 3 illustrates noise test results after the gear pair was ground further from the condition in which the results in FIG. 2 were obtained. The periodic noise bands are clearly distinguishable on the thrust side and the second harmonic exceeds the permissible threshold noise level at 635 Hz.
The results shown in FIG. 4 were obtained after the gears were lapped. The scatter band noise levels were dramatically reduced. However, the lapping step requires, inter alia, increased machining time, and thereby cost. While the same results can be obtained by finish grinding the gears, this too increases machining times and cost. Another problem associated with lapping results from the abrasive materials sprayed between the gear teeth. During the lapping process, grains of this hard abrasive material become embedded into the gear flank surfaces due to the high pressures generated between the gears. These particles often remain embedded in the finished gears, and can cause excessive wear during operation. In addition, in automotive applications, these particles can become dislodged from the gear surfaces and be carried to bearings by engine lubricating oil where permanent bearing damage can occur. The resultant bearing wear can cause improper gear tooth meshing and increased noise generation.
Another manner by which bevel gear pairs have been machined is disclosed in German patent number DE 34 25 800 A1 whereby the gear tooth flanks are honed after heat treatment. A drawback associated with this method is that the bevel gears must be pre-treated and hardened extremely accurately so that the post-hardening distortion is kept to a minimum. This is due to the fact that honing removes material very slowly and is expensive so that large distortion due to heat treatment would be time consuming and expensive to correct.
Another manner by which bevel gear pairs are finished is set forth in patent number DE 38 26 029 C2 and consists of finishing the gear teeth after hardening by strip hobbing the teeth of one of the gears and grinding the teeth of the other. Strip hobbing basically consists of post-hardening milling of the gear teeth. The use of two different machining operations is thought to achieve lower operating noise levels by employing a gear pair where one gear has a comparatively smooth flank and one a rough surface. However, since the above-described strip hobbing and grinding is done after hardening, extra machining operations as well as expensive hardened tools are required. In addition, because the extent of distortion after hardening is unpredictable, repeatability from gear pair to gear pair is uncertain.
Based on the foregoing, it is apparent that the noise characteristic of a bevel gear pair is substantially determined by the geometric meshing relationship between the ring gear and pinion. Accordingly, it is the general object of the present invention to provide a bevel gear pair that overcomes the problems and drawbacks of prior art gears.
It is a more specific object of the present invention to provide a bevel gear pair capable of operation within acceptable noise limits without the necessity of maintaining and using the gears in matched sets.
To allow for a detailed analysis of the noise behavior of a bevel gear pair, the micro-geometry in the contact zone between the flanks of meshing gears must be considered. To accomplish this, two gear flank surface characteristics need to be recognized, namely, the surface texture including microstriations, and surface micro-undulation. As used herein, the term microstriation refers to successively oriented machining marks that appear as scratch-like marks on the gear flank. The microstriations are formed by grinding, honing or like single direction material removal operations.
The term micro-undulation refers to surface irregularities created by movement of a grinding tool during a grinding operation.
The present invention resides in a bevel gear pair comprising a first gear having a plurality of first gear teeth, each defined at least in part by a tooth root, and corresponding convex and concave flank surfaces. The flank surfaces of the first gear each define a first surface structure formed by grinding. This operation creates microstriations extending diagonally across each concave and convex gear flank surface. A second gear includes a plurality of second gear teeth adapted to mesh with the teeth of the first gear. Each of the plurality of second gear teeth also defines a root, and a concave and convex flank surface. The convex and concave flank surfaces of the second gear define a second surface structure formed by grinding followed by honing. The topography of the second surface structure is defined by microstriations extending across each flank surface approximately parallel to the respective gear tooth root. During operation, as the first and second gears mesh together, the microstriations of the flank surfaces of the gears are offset relative to each other such that the noise frequency generated by each gear tends to cancel the noise frequency generated by the other gear. These offset microstriations allow unmatched gear pairs to operate within acceptable noise limits.
An advantage of the present invention is that with gears made in the above-described manner, it is no longer necessary to store a ring gear and pinion together as a matched set.
Another advantage of the present invention is that the grinding process need not be performed with the accuracy required by the prior art, thereby reducing manufacturing time and cost.